Merge branch 'jg/credential-cache-chdir-to-sockdir'

The "credential-cache" daemon process used to run in whatever
directory it happened to start in, but this made umount(2)ing the
filesystem that houses the repository harder; now the process
chdir()s to the directory that house its own socket on startup.

* jg/credential-cache-chdir-to-sockdir:
  credential-cache--daemon: change to the socket dir on startup
  credential-cache--daemon: disallow relative socket path
  credential-cache--daemon: refactor check_socket_directory

Documentation/git-credential-cache.txt

@@ -36,7 +36,7 @@ OPTIONS
 	cache daemon if one is not started). Defaults to
 	`~/.git-credential-cache/socket`. If your home directory is on a
 	network-mounted filesystem, you may need to change this to a
-	local filesystem.
+	local filesystem. You must specify an absolute path.
 
 CONTROLLING THE DAEMON
 ----------------------

credential-cache--daemon.c

@@ -215,7 +215,7 @@ static const char permissions_advice[] =
 "users may be able to read your cached credentials. Consider running:\n"
 "\n"
 "	chmod 0700 %s";
-static void check_socket_directory(const char *path)
+static void init_socket_directory(const char *path)
 {
 	struct stat st;
 	char *path_copy = xstrdup(path);
@@ -224,20 +224,27 @@ static void check_socket_directory(const char *path)
 	if (!stat(dir, &st)) {
 		if (st.st_mode & 077)
 			die(permissions_advice, dir);
-		free(path_copy);
-		return;
+	} else {
+		/*
+		 * We must be sure to create the directory with the correct mode,
+		 * not just chmod it after the fact; otherwise, there is a race
+		 * condition in which somebody can chdir to it, sleep, then try to open
+		 * our protected socket.
+		 */
+		if (safe_create_leading_directories_const(dir) < 0)
+			die_errno("unable to create directories for '%s'", dir);
+		if (mkdir(dir, 0700) < 0)
+			die_errno("unable to mkdir '%s'", dir);
 	}
 
-	/*
-	 * We must be sure to create the directory with the correct mode,
-	 * not just chmod it after the fact; otherwise, there is a race
-	 * condition in which somebody can chdir to it, sleep, then try to open
-	 * our protected socket.
-	 */
-	if (safe_create_leading_directories_const(dir) < 0)
-		die_errno("unable to create directories for '%s'", dir);
-	if (mkdir(dir, 0700) < 0)
-		die_errno("unable to mkdir '%s'", dir);
+	if (chdir(dir))
+		/*
+		 * We don't actually care what our cwd is; we chdir here just to
+		 * be a friendly daemon and avoid tying up our original cwd.
+		 * If this fails, it's OK to just continue without that benefit.
+		 */
+		;
+
 	free(path_copy);
 }
 
@@ -264,7 +271,10 @@ int main(int argc, const char **argv)
 	if (!socket_path)
 		usage_with_options(usage, options);
 
-	check_socket_directory(socket_path);
+	if (!is_absolute_path(socket_path))
+		die("socket directory must be an absolute path");
+
+	init_socket_directory(socket_path);
 	register_tempfile(&socket_file, socket_path);
 
 	if (ignore_sighup)